Laser radars have performance advantages over microwave radars in certain applications, primarily because of their high angular resolution. However, to search a large field of view with a narrow laser beam in a short time requires a fast, large aperture scanner. Such a scanner is often the component which limits system performance, and the absence of suitable scanners prevents consideration of laser radars for many applications.
To eliminate the need for a scanner while still efficiently utilizing the source power, a detector array can be used in conjunction with a laser which illuminates the entire scene. Square detector arrays of the integrating type, with hundreds of elements in each dimension have been developed for this application. They convert each incoming photon into an electrical charge and store this charge until it is read out. Such detectors are suitable for forming an intensity image. However, the determination of range to a target requires that the laser be modulated and that this modulation be detected. The integrating type of detector does not preserve the intensity modulation of the received optical signal. These considerations indicate why it has heretofore been difficult to fabricate a range imaging laser radar which does not have a scanner.
Nonscanned range imaging laser radar configurations exist in the prior art. In U.S. Pat. No. 3,866,052, differently configured masks establish diverse irradiated segments of the projection field and are used successively. Records made in corresponding succession are examined to identify those records in the succession of records which contain or do not contain a representation of the illumination of each surface point of interest. Digital signal patterns are generated from the record succession, unique for surface points which are in different projection field segments. These signals, together with information as to the positional location of surface points in the two-dimensional records and the spatial location of the recording lens-mode enable transition from two-dimensional data to special position determination. The lens mode and record positional information establish a line of sight to the surface point and the digital signals identify the location of such point along the line of sight.
In U.S. Pat. No. 4,259,017, radiant energy is projected into the projection field with the intensity, or other characteristic, of the radiant energy conforming successively per projection, to plural different patterns, e.g., sine, cosine, or like functions, along an axis transverse to the direction of the projection field. Records are made of object-reflected radiant energy and the line of sight distance to the object surface point is derived from the records.
In U.S. Pat. No. 4,199,253, codes are used to encode the round-trip travel time of a light pulse to and from an object. In one embodiment, plural shutters are cycled in accordance with respective different time patterns by operating the shutters at respective different rates in a common time frame, following issuance of a pulse of light onto an object. The shutter-cycling rate and timing of mutual operation of shutters are controlled such that energy reflected from a plurality of zones of the object is conducted selectively through participating shutters. In another embodiment, a single shutter is employed and is cycled in accordance with respective different time patterns by operating the shutter at respectively different rates following issuance of successive light pulses, cycling rates being such that range information is attained.
Others have proposed the use of a pulsed laser source in conjunction with a gated receiver, much the same as in well known lower frequency, pulsed radars. Implementation of such gated systems is very costly and more practical alternatives are needed.
Accordingly, it is an object of this invention to provide an improved, nonscanning laser radar.
It is a further object of this invention to provide an improved receiver particularly suited for use in a nonscanning laser radar.
It is another object of this invention to provide a nonscanning laser radar with improved range imaging.